Sheathed heating element and process of making the same



1960 s. A. OAKLEY 2,947,963

' I SHEATHED HEATING ELEMENT AND PROCESS OF MAKING THE SAME Filed April 8. 1958 INVENTOR.

staid? 04% ddomeyAJ SHEATHED HEATING ELEMENT AND PROCESS OF MAKING THE SAME Sterling A. Oakley, 6831 N. Kilpatrick Ave.,

Lincolnwood, Ill.

Filed Apr. 8, 1958, Ser. No. 727,101

7 Claims. (Cl. 338-278) The present invention relates to sheathed electric heating elements and the process of making the same.'

The so-called sheathed type of electric heating elements has been used extensively for many years. The great bulk of the heating elements for electric ranges and modern appliances, such as pressing irons, cooking vessels and cotfee makers, are now of the sheathed type. Such sheathed heating elements comprise an outer sheath of a nickel copper alloy or the like for enclosing therein an electric resistance heating element, usually in the form of a wire wound in the form of a helical coil, with the longitudinal axis of the coil disposed on the longitudinal axis of the sheath. This resistance heating element is generally formed of a nickel chromium alloy which may be purchased on the market under the trademark Ni chrome. The space within the sheath not occupied by the coiledresistance wire is filled with a material which is both a good electrical insulator and a reasonably good conductor of heat. A compacted fused magnesium oxide is generally employed for this purpose. The insulating material within the sheath usually is compacted by suitable methods such as swaging or rolling the sheath after the wire and insulating material are disposed therein.

When such a sheathed heating element is used in any application, it is, of course, necessary to make electrical connections with the resistance element within the sheath, and in manufacturing such sheathed heating elements electrical terminals are normally connected one to each end of the resistance element, which terminals then project "from'the ends of the sheath. One method of manufacturing such sheathed heating elements is disclosed and claimed in my copending application Serial No. 580,573, filed April 25, 1956. a

' Depending upon the particular application, such sheathed heating elements are manufactured in various sizes; Today there are employed sheathed heating elements which have 'an outside sheath diameter of from two hundred fifty thousandths of an inch to an inch or 'more. It is, of'course, desirable to get a high wattage density. It the wattage density of the heating element is made too great, it may burn out. This watt-age density depends upon how fast the heat produced by the electrical resistance heating element isidissipated through the insulating material and the sheath. Generally there is a temperature gradient of the order of 300 F. between the outside of the sheath and the temperature of the resistance element. Obviously, if the temperatureof the resistance element is too high, it will fuse, which is another way of saying that it will burn out.

It is, of course, important that the resistance element be accurately centered within the sheath so that short circuits do not occur between the heating element and the sheath. As a matter of fact, in orderto pass certain test requirements, such sheathed heating elements must be able to withstand relatively high voltages between the resistance and the sheath. This, of course, introduces some limitations as to how close the heating element can United States Patent In producing such a sheathed heating element, it is common practice to take the resistance wire and wind it into a helical coil on a suitable mandrel to a predetermined outside diameter of the coiled wire to be disposed within the sheath- After the terminals are secured to the coiled resistance wire, a tensile force is applied to the terminals so as to provide the desired separation between adjacent turns of the coil. It will be appreciated that there are limitations on the cross-sectional area of the resistance wire employed, which depends upon the wattage density involved. There also are mechanical limitations to the interior diameter of the coiled resistance element. However, the space within the center of the coiled resistance element is pretty much waste space and not only increases the size of the sheath but, consequently, increases the cost, since the materials making up a sheathed heating element are relatively expensive. Furthermore, a predetermined spacing between adjacent turns of the coiled resistance heating element is required,

and this spacing determines the overall length of the sheathed heating element to produce a certain heating operation. It would be desirable to provide a heating element where the same wattage density could be produced by employing less materials for making all three elements of the sheathed heating element, namely, the

outer sheath, the coiled resistance element, and the insulating and spacing material commonly comprising fused magnesium oxide than was heretofore required.

The problem of securing suitable terminals to the ends of the coiled resistance element has been a difiicult one, and many solutions have been suggested. One solution is disclosed and claimed in my copending application Serial No. 525,040, filed July 28, 1955, now Patent No. 2,890,320.

It would also be desirable to provide an improved arrangement of securing terminals to the electrical heating element as well as the improved arrangement for reducing the cost of manufacture of such heating element.

Accordingly, it'is an object of the present invention to provide a new and improved process of manufacturing a be tq the sheath. In the copending application referred sheathed heating element.

It is another object of the present invention to provide an improved sheathed heating element which is substantially lower in cost and yet equally eifective as a heating element.

It is another object of the present invention to provide an improved method of securing terminals to the ends of the coiledresistance element.

Further objects and advantages of the present invention will become apparent as the following description proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the present invention, reference may be had to the accompanying drawing in which:

Fig. 1 is a perspective view of a portion of a sheathed heating element embodying the present invention;

Fig. 2 is a greatly enlarged fragmentary, longitudinal sectional view of the heating element of Fig. 1;

Fig. 3 is a sectional view illustrating one step in the process of manufacturing the heating element of the present invention;

Fig. 4 is a view similar to Fig. 3 showing a slightly later stage during the step of manufacturing the heating element of the present invention disclosed in Fig. 3;

Fig. 5 is a sectional view taken on line 5-5 of 2; and

Figs. 6 and 7 are fragmentary views, somewhat schematic in nature, illustrating another step in the process of manufacturing the heating element of the present invention.

Briefly, the present invention is concerned with an improved sheathed heating element and also an improved method of manufacturing the .coiled resistance ele- .ment including the terminals thereoffor a sheathed heating element. The coiled resistance element is produced in conventional fashion, but this element then has a comtotal length of the resistance element may be shortened substantially. Additionally, the flattening will not result in any increase in the outer diameter of the coiled resistance element, but will'result in a considerable reduction in the inner diameter of the coiled element. Thus, the heating element, when manufactured in accordance with the present invention, effectively comprises a flat wire strip edge wound so that the narrow dimension of the cross section of the wire is parallel with the longitudinal axis of the heating coil, and the wide dimension is perpendicular to the longitudinal axis. It will be appreciated that to obtain such a coil by a winding process would be very diflicult, but by the process of the present invention it is very simple indeed. Since the flattened helix presents a plane surface at each end, a terminal connection can be obtained by merely abutting the end of a terminal having the same outer diameter as the coiled resistance element against this flat end of the coil. By applying an electric current to the terminal and a portion of the coil, and then separating the terminal from the coil, an arc can be drawn causing melting or softening of the corresponding end of the coil and the end of the terminal, which then can be pressed together, with a resultant weld.

Referring now to the drawing, there is illustrated a sheathed heating element generally designated at 10, comprising an outer sheath 11 within which there is disposed a coiled resistance heating element 12, which coiled resistance heating element has its longitudinal axis disposed on the longitudinal axis of the sheath 11. The space within the sheath 11 not occupied by the heating element 12 is filled with a suitable material, such as granular magnesium oxide, generally designated at 13. It will be understood that the assembly of the sheath 11, the coiled resistance heating element 12 and the filler material 13 can be accomplished exactly in the manner disclosed in my copending application Serial No. 580,573 referred to above, and the particular method of assembly forms no part of the present invention. It will also be appreciated that, in accordance with normal manufacturing procedures with respect to such sheathed heating elements, the completed assembly of sheath 11, heating element 12 and filler material 13 is swage'd or rolled so that the exterior diameter of the sheath 11 is reduced to some extent, thereby eliminating any voids which might exist within the filler material 13.

As illustrated in Fig. l of the drawing, a suitable terminal 14 is electrically connected to the adjacent end of the resistance element 12 and projects from the end of the sheath 11. A portion of such terminal 14 is also indicated in Figs. 6 and 7 of the drawing and will be described in greaterdetail hereinafter.

By virtue of the present invention, as will be described in detail hereinafter, the sheath 11 may have a smaller outer diameter than sheaths heretofore employed Without sacrificing in any way on the heat output of such a heating element, and this is accomplished by flattening each turn of the coiled heating element in the direction of'the longitudinal axis. In effect this action reduces the dimension of each turn in the direction of the longitudinal axis and increases the dimension in a direction perpendicular to the longitudinal axis, but in the latter case ,without increasing the outer diameter of the coil resistanceelement, thus utilizing some of the space within the edgewise.

4 coiled resistance element which heretofore was wasted completely.

In accordance with the present invention, the sheath 11 may be exactly the same as in the prior art devices except that it can comprise tubing of smaller diameter for the same rated heating element. duces a substantial saving, since the sheaths formed of a nickel copper alloy comprise relatively expensive material. Likewise, the coiled resistance element is similarly formed as in the prior art heating elements, namely, a circular cross section wire suchas is coiled on a mandrel so as to produce a helical coil of Wire with every turn immediately engaging an-adjacent turn, as is designated at 12 in Fig. 3 of the drawing. The outer diameter of this coiled resistance wire, preferably formed of a nickel chromium alloy, is chosen with respect to the inner diameter of the sheath 11 to provide the necessary clearance which is required to withstand the high voltage requirements of the manufacturer. In accordance with the present invention, this coiled resistance element 12 is then compressed longitudinally in any suitable manner so that the engaging surfaces of adjacent turns are flattened, and so that the coil 12' of Fig. 3 is effectively of the configuration shown in Fig. 4 and designated by the reference numeral 12". It will be apparent from Fig. 4 that this results in effectively producing a coil in which a flat strip is wound The outer diameters of the coils 12' and 12" are effectively the same, but the inner diameter of the coil 12" is substantially smaller than that of the coil 12'.

As was mentioned above, the flattening step of the present invention may be carried out in numerous ways.

For instance, the coil 12 may be inserted into a suitable forming die 16 comprising an elongated cylindrical member having an inner diameter of the order of the outer diameter of the coiled resistance element 12'. The forming die .16 may be closed at one end as indicated at 16a. A suitable plunger 17 is adapted to enter one end of the forming die .16 in the manner of a piston and, under the influence of a suitably hydraulic or pneumatic force, the

plunger 17 will cause the coil 12 to be reduced substantially in length and be converted to the coil 12" shown in Fig. 4 of the drawing. in fact, Figs. 3 and 4 together show the compression stage of the present invention resulting in the coil '12", each turn of which has the identical cross sectional area of the coil 12 shown in Fig. 2 of the drawing. 7

It will be appreciated that instead of using a single plunger 17 with the forming die 16 having a closed end 16a, the forming die 16 may 'be open at both ends and two hydraulically or pneumatically actuated plungers, one from each end, may be employed to convert the form of the coil 12 to that of the coil 12". The specific apparatus for accomplishing this forms no part of the present invention. It will be appreciated, however, that this step results in a simple means of obtaining a coiled strip of resistance material, with the wide dimension of the strip perpendicular to the longitudinal axis of the helical coil, and wherein the inner diameter of the helical coil is substantially less than it was before the flattening step occurred. This results in utilizing more of the space within the sheath and permits a smaller outer diameter for the same heating element.

It will be apparent that each end of the helical coil 12" is flattened in a single plane, which is very desirable from the standpoint of securing a terminal thereto. In accordance with the present invention, a terminal 14 having the same exterior diameter as the coil 12" is employed, and the adjacent ends of the terminal 14 and coil 12" are abutted as indicated in Fig. 6 of the drawing. To make a good electrical connection, a suitable source of welding current, generally designated at .20, is provided, one terminal being connected by a conductor 21 with the coil '12", and the other terminal 20 being connected by a conductor 22 with the terminal 14, as indicated in Fig. 7 of the drawing. Welding current is permitted to flow, and the coil This, in itself, pro-' 12" and the terminal 14 are momentarily separated as indicated in Fig. 7 to draw an are 23 therebetween. This, of course, causes sufficient heating of the formerly abutting ends of the coil 12" and terminal 14, and these two members are immediately pushed together to the position shown in Fig. 6 of the drawing following this drawing of an arc, with the result that they are securely butt-welded together.

A similar terminal 14 is applied to the other end of the resistence element and it is now ready for incorporation in a suitable sheath such as 11. Prior to this step, however, the coil is stretched so as to separate the individual turns the desired amount. An important advantage of the present invention readily becomes apparent here. Due to the fact that the wire forming the heating element has been flattened in the direction of the longitudinal dimension of the coil, the separation between centers of adjacent turns can be less without reducing the separation between adjacent portions of adjacent turns. Thus, if the wire is flattened so as to reduce its dimension in the longitudinal direction of the coil by only two thousandths of an inch, a one thousand turn coil would be two inches shorter, thereby providing a substantial saving in material without even considering that a smaller diameter sheath may be employed due to utilizing more of the space within the turns of the helix. In actual practice it has been found a very simple matter, in going from Fig. 3 to Fig. 4, to reduce the over-all length of the coiled resistance element between 30% and 40%. This results in a very substantial saving in materials, which is the expensive part of resistance heating elements of the sheathed type which are manufactured primarily by automatic machinery.

In view of the detailed description included above, the process of the present invention and the improved heating element produced thereby will readily be understood by those skilled in the art and no further description is included herewith.

While there has been illustrated and described a preferred embodiment of the present invention, including not only the heating element per se but the process of manufacturing the same, it will be understood that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A sheathed type electric heating element comprising an outer metal sheath, a helical resistance element within said sheath with the longitudinal axis thereof coincident with the longitudinal axis of said sheath, and a granular material filling all the space within said sheath not occupied by said resistance element, said helical resistance ele ment comprising a wirelike element, said element having a narrow cross section in the direction of said longitudinal axes and a wider cross section in a direction perpendicular to said longitudinal axes.

2. The heating element of claim 1 wherein each end of said helical element defines a flat surface, and terminal means abutting said surface and electrically and mechanically connected thereto. 1

3. The method of making an electrical resistance element for use in a sheathed type electric heating element applying a force to said coil to shorten the same in the direction of its longitudinal axis without substantially Y changing said predetermined outer diameter but suhstantially decreasing said predetermined inner diameter, said wire defining each turn of said helical coil being reduced in the dimension thereof parallel to said longitudinal axis.

4. The method of making an electrical resistance element for use in a sheathed type electric heating element which comprises winding a resistance wire of circular cross section into a longitudinally extending helical coil of a predetermined outer and inner diameter with adjacent turns of said coil in contacting engagement, applying a force to said coil to shorten the same in the direction of its longitudinal axis without substantially changing said predetermined outer diameter but substantially decreasing said predetermined inner diameter, said wire defining each turn of said helical coil being reduced in the dimension thereof parallel to said longitudinal axis, applying a terminal to each end of said coil, and stretching said coil to provide a predetermined separation between each turn.

5. The method of making a sheathed type electrical heating element which comprises winding a resistance wire of circular cross section into a longitudinally extending helical coil of a predetermined outer and inner diameter with adjacent turns of said coil in contacting engagement, applying a force to said coil to shorten the same in the direction of its longitudinal axis without substantially changing said predetermined outer diameter but substantially decreasing said predetermined inner diameter, each turn of said helical coil being reduced in the dimension thereof parallel to said longitudinal axis, applying a terminal to each end of said coil, stretching said coil to provide a predetermined separation between each turn, inserting said helical coil within a metal sheath, and filling the space within said sheath with a granular insulating material in a manner to center said coil along the longitudinal axis of said sheath with said terminals projecting slightly one from each end of said sheath.

6. The method of claim3 wherein each end of said coil has a terminal secured thereto by butt welding.

7. The method of claim 3 involving bringing a terminal into abutting relationship with one end of said coil, connecting a source of electric welding current to cause such current to flow between said coil and said terminal, separating the end of said coil and said terminal slightly to draw an arc therebetween to heat said end and said terminal, and then moving the end of said coil and terminal into abutting relationship to weld them together.

References Cited in the file of this patent UNITED STATES PATENTS France Nov. 24, 1938 

